Addition of rctt-tetrakis(2-pyridyl)cyclobutane (2,2'-tpcb) in a Cu-II/N-3(-) solution afforded the 1D coordination polymer [Cu-3(N-3)(6)(2,2'-tpcb)(DMF)(2)](n) (1). The ligand 2,2'-tpcb serves as a tetradentate bis-chelating ligand by linking linear [(DMF)Cu(mu(1,1)-N-3)(2)Cu(N-3)(2)(mu(1,1)-N-3)(2)Cu(DMF)] trinuclear units to produce a zigzag chain. Within each centrosymmetric trinuclear unit there exist two irregularly asymmetric end-on double azido-bridged [Cu(mu(1,1)-N-3)(2)Cu](2+) cores, while one of the largest Cu-N-azide-Cu angles is observed. Magnetic susceptibility data, measured from 2 to 300 K, show bulk moderate ferromagnetic coupling within the magnetically isolated trinuclear units. These data were fitted to the appropriate equation derived from the Hamiltonian H = -J(1)(SA1SB + SA2SB) - J(2)S(A1)S(A2), giving the parameters J(1) = +70(3) cm(-1), J(2) = -3(2) cm(-1), g = 2.12(1), with an intertrimer interaction parameter theta = -0.74(2) K. The coupling constants were correlated with the structural parameters. Density functional calculations reproduce very well the experimental J values and show that ferromagnetism for this complex is mainly due to the topology of the magnetic orbitals and the different coordination spheres of two neighboring Cu-II atoms, resulting in a small overlap of the orbitals possessing the unpaired electrons.